Conventional methods of spinal fixation utilize a system comprising a set of pedicle screws and a set of rigid, metallic rods to stabilize one or more vertebra. Permanent immobilization of one or more functional segmental units (FSU) is the desired clinical outcome for such procedure.
Although stabilization of the spine is the main objective of the fixation, immobilization of the spine with stiff, non-compliant bars or rods is known to have adverse side effects. Among them, stress shielding and changes in the loading patterns on the facets and other supporting spinal structures have been reported.
One of the reasons titanium is often selected over stainless steel as the rod material is its lower elastic modulus. Having a lower stiffness allows the titanium rod to bend and flex a little more than its stainless steel counterpart, somewhat limiting stress shielding and sparing the facets (although not by a great measure). Thus, it must be recognized that the biomechanical advantage of the titanium rod is minor and consequently the need for a more compliant system is not truly addressed.
Therefore, to further provide limited mobility to the FSU, reduce stress shielding, and reduce unwanted loads on the supported spinal structures, a more drastic approach than a simple change in the material composition is needed.
To solve the above-described problems associated with rigid fixation, dynamic stabilization devices have been developed. Although the majority of these devices provide added flexibility, their applicability can be limited due to the shortcomings in their spring design, with the majority providing added compliance in flexion-extension but lacking torsional stiffness, a pre-requisite for a well-controlled stability.
In order to mimic the physiologic spine, rods having spring components must provide the appropriate stiffness in flexion-extension, lateral bending, and compression-distraction. To do so, the spring system must have individually tuned translational and rotational springback properties.
A well-designed dynamic system should reflect a compromise between stiffness and compliance—not so stiff as not to load the adjacent structures, but not so compliant as to fail to provide the required stabilization. Thus, it is an object of the present invention to provide a mechanism for harmonious load-sharing between the biological structures.
Examination of prior art devices reveals shortcomings in attaining this goal:
U.S. Published Patent Application Numbers US20040049190A1 (“Biedermann I”), US20050085815A1 (“Harms I”), and US20050154390A1 (Biedermann II) suggest that the elastic section of the rod “be implemented in the form of a helical spring”. A similar device is disclosed in U.S. Published Patent Application Number 20050203517A1 (“Jahng”). These devices are flexible, but they are not well-suited for resisting lateral forces or torsional moments.
U.S. Published Patent Application Numbers US20050288670A1 (“Panjabi”) discloses a dynamic stabilization device “including overhanging stabilizing member”. However, this device is cumbersome and complex, requiring several individual parts for the fabrication of a “shock absorber like” spring. The benefits of the device are limited to translational flexibility.
U.S. Published Patent Application Numbers US20040002708A1 (“Ritland”) discloses a novel dynamic fixation device wherein the rod has a ring provided therein. However, this patent document is primarily concerned with providing structural support that “limits the amount of translation motion beyond normal physiological limits”. Moreover, the large aspect ratio of the ring has the potential for impinging on surrounding tissues and may present challenges to the surgeon who desires to minimize harm to soft tissues (such as muscles and the like).
U.S. Published Patent Application Number US20050203519A1 (“Harms II”) discloses a rod-shaped element that allows for a controlled motion of the parts to be stabilized relative to each other so that the “flexural motion is adjusted separately from the adjustment of the mobility in the axial direction”. However, this device falls short by failing to include a mechanism for controlling rotational stiffness, which, if not properly selected, may prevent the device from functioning flawlessly.
U.S. Pat. No. 6,267,764 (“Elberg”) discloses spine stabilization system having a pair of pedicle screws and a rod having an open ring therein. This design has the disadvantage in that the open nature of the ring does not adequately resist torsion.